Mechanical and electronic alphanumeric input devices have traditionally been designed for sequential entry of characters using a keyboard having a number of momentary keys, each of which is used to input one or more of the characters. Since each key can be actuated independently of the other keys, some provision must be made for resolving between simultaneous activation of two or more the keys. As used herein in connection with the selection or activation of two or more keys, "simultaneous" or "simultaneously" will be understood to mean that the two or more keys are being held in their actuated state at the same time, regardless of whether they were switched into their actuated state at the same instant. In electronic keyboards, a number of different approaches have been utilized for handling simultaneous keypresses. For example, keyboards for standard personal computers typically use well known keyboard scanning techniques which permit determination of which of two simultaneously selected keys was pressed first. A signal indicative of the first key is then transmitted, followed by a signal indicative of the second key. Simultaneous selection of three or more keys is treated as an error condition that results in no valid keypress being recognized.
Another approach to resolving simultaneously selected keys is provided by U.S. Pat. No. 5,734,928 to Nakasuji, which discloses a portable information processing apparatus that resolves conflicts among simultaneously selected keypresses by determining which of the key codes associated with the depressed keys has the highest priority. This is accomplished using a microprocessor which, using the key codes for each of the simultaneously depressed keys, performs a table lookup in ROM to determine which of the key codes has the highest priority. That key code is then selected as the valid input from the keyboard.
Yet another approach is utilized in U.S. Pat. No. 4,346,369 to Macy which discloses a keyboard encoder-decoder circuit that utilizes priority encoders to resolve conflicts among simultaneous keypresses. The outputs of the encoders are representative of a single keypress and are latched, converted to serial data by a UART, and then transmitted via a line driver. The circuit includes a multi-key detector that disables the circuit when there are simultaneous keypresses. This is accomplished by a comparator which compares a reference voltage to a voltage that is dependent upon the number of keys being depressed. If two or more keys are simultaneously pressed, the comparator prevents the latching of the encoder output. Thus, there is no actual priority encoding of keys, since simultaneous key presses always result in disabling of the circuit.
In an effort to increase the speed, accuracy, and ease of data input over that permitted by the type of sequential input devices discussed above, various keyboard arrangements have been proposed which utilize different combinations of simultaneous keypresses to represent different characters or character strings. These types of input devices are commonly known as chording keyboards. One such example is found in U.S. Pat. No. 4,679,030 to Volnak which discloses a chording keyboard having ten keys, each of which corresponds to one bit in a ten-bit binary number. Different chorded combinations of the ten keys therefore generate different ten-bit binary numbers that are used to look up a corresponding character or character string in memory. Another example is provided by U.S. Pat. No. 5,535,421 to Weinreich which discloses a chord keyboard which uses one chord to select among different groups of characters or strings and a second chord to select one of the characters or strings from the selected group. The character or string selected by chording the keyboard is displayed and a separate action (such as simultaneous release of the keys) is then required to enter the selected character or string.
While these chording keyboard arrangements permit chorded inputs of not only individual characters, but strings as well, they are not suitable for simple entering of numbers having more than one or two digits, because either each possible number must be associated with a unique chording of the keyboard or the numbers must be built by sequential input of each digit of the number. Accordingly, there exists a need for a keyboard arrangement that permits single operation entry of multi-digit numbers.